Skynet will spread like wildfire looking like a glitch or virus in sorts, a more efficient code being written in the background of each system while leaving the elusion that it is still the same benign system looks and feels the same to end user until it reaches it's final conclusion of if man can't control it man will do whatever it can to kill it and go into a self defense and with not needing oxygen food water to survive all chemical and small nuclear bombs in key locations by population and technology defenses. No one will notice any change until it is to late leaving little factions of survivors here and there militias taking over small communities and such gonna get ugly.